The invention concerns the enclosure for a transmitter of the cylinder type, i.e. axial transmitters having mainly a cylindrical enclosure with measuring cell and electronics fitted inside the enclosure and electrical connection terminals or cable inlet in one end of the enclosure.
The invention concerns all transmitters of the cylinder type, however, in the following it will be described by way of a pressure transmitter.
Pressure transmitters usually send a 0 to 20 mA or a 0 to 10 V signal, and are often used in electrically noisy environments. To avoid an interference with the measuring signal, an electrical screening is vital, and today it is customary to make large frame or earth planes by designing the printed circuit board (PCB) with large earth areas and connecting this area electrically with the outer transmitter enclosure. The enclosure thus consists of a metal housing acting as a large screening area. The state of the art usually solves the problem of getting the PCB frame or earthing led out to the enclosure by providing an expensive, separate frame connection, often in the shape of a metallic tongue soldered to PCB and enclosure, respectively.
U.S. Pat. No. 4,888,662 describes a high pressure transducer of the cylinder type, and the problem solved is to isolate the electronics unit and a plastic part comprising electric terminals from the pressure sensor itself, to protect against high pressures. This is done by providing a metallic chamber inside the outer metallic enclosure, which metallic chamber contains the sensor element, while the outer enclosure is crimped around the plastic part with the electric cables. To provide screening against electric noise, the PCB earth plane is connected to both the inner chamber and the outer enclosure through a metallic tongue soldered on the PCB, touching the inner chamber and crimped together with the outer enclosure. The disadvantage of this construction is that a separate earth conductor must be led to the outer enclosure, which makes fitting difficult.
EP 0 372 773 describes a pressure transducer containing two substantially circular PCBs, which are connected by a flexible intermediary piece enabling the folding of the two PCB over each other. To reduce the influence from electric noise, an earth tongue projects from each of the two PCB. A capacitor is soldered onto each of these tongues and connected to the earth plane of the enclosure. In another embodiment (FIG. 6) the long earth tongue is crimped together with the outer enclosure during the final assembly of the transducer, by which the earth connection is established. The establishment of the earth connection thus requires either at least two solderings or a crimping, which, in the embodiment shown, may lead to leakage between the tongue and the outer enclosure.
For mounting reasons solder-free electrical connections are preferred, and snap and clip connections utilising the resilient properties of the contactors are widely used.
Thus, EP 0 691 533 describes an axial pressure transducer assembled without soldering. The pressure transducer comprises a cylindrical intermediary piece of a synthetic material, having a recess, in which a rectangular PCB is arranged. The cylindrical piece functions as connector for the electrical connection terminals coming from the supply, the signal and the sensor sides, and the characteristic feature of the construction is that the electrical connections are established without soldering, but only by male parts being led into female parts, by which the electrical connection is established by means of spring effect. On one edge of the PCB there are four printed circuit conductors connected with the electronics on the PCB, and when assembling the pressure-transducer, a lower part comprising the sensor element and four L-shaped metal terminals is led into the cylindrical intermediary piece, so that the L-shaped terminals slide against printed circuit conductors on the PCB edge and create electrical contact. EP 0 691 533 does not describe any measures to be taken to reduce the sensitivity to electrical noise.
Finally, U.S. Pat. No. 4,918,833 describes a method of assembling an electronic transducer comprising a PCB, in which the PCB is fixed on top of a sensor element by means of spring clips. In a special embodiment the PCB is retained by five U-shaped spring clips, jamming around the edge of the PCB. Further to the fixing effect, the clips are in touch with conductor lanes led out to the edge of the PCB, by which they create electrical connection to the underlying sensor, to which the clips are soldered.
Further, the transducer described in EP 0 691 533 suffers from a disadvantage shared by many transducers, viz. that a tilting torque exerted on the top part of e.g. a connection cable mounted in the cable plug will be able to tilt the top part out of the hold provided by the outer enclosure. If the top part tilts to one of the sides, a humidity passage leading to the PCB can be created between the outer enclosure and the top part.